1. Field of the Invention.
The present invention relates to imaging systems for use in imaging anatomical structures undergoing high energy irradiation such as that used in radiation teletherapy.
2. Description of Related Art.
Various imaging systems have been developed and used in medical diagnostic procedures, ranging in sophistication from basic x-ray film techniques to computerized tomography (CT) scanning techniques. These diagnostic procedures employ a low level of energy in irradiating the human body in order to avoid the harmful side effects experienced with high doses of radiation. Imaging is relatively easy to accomplish in such low energy irradiation systems, as the detection portions of such systems are capable of producing sufficiently high contrasts between various objects in the path of the radiation beam.
Notwithstanding the advances in the field of low energy diagnostic imaging, a problem has long existed in developing imaging systems for use with high energy irradiation, such as that used in radiation therapy, to ensure proper treatment or provide improved methods of treatment of certain cancer patients. Energy levels used in radiation therapy are generally greater than 1 million electron volts (MeV), and may typically range from 4 to 25 MeV. Proper treatment requires that an optimum level of radiation be directed at a specific portion of the patient's body, while at the same time minimizing the exposure of all other parts of the body to the radiation. The irradiated area for which it is desired to obtain images is commonly referred to as a radition target or treatment target. The entire field being irradiated is generally referred to as a radiation or treatment portal.
Various methods, systems and techniques have been employed since the early days of radiation therapy in order to effectuate the minimization of the harmful side effects of the high energy irradiation of the body. One technique for minimizing the harmful effects employs small, tailored radiation fields or portals in what is sometimes referred to as "customizing" the treatment.
There are also systems known which attempt to improve the quality and consistency of the radiation treatment sessions by using what is sometimes referred to as a "record and verify" system. The "record and verify" systems conduct system checks to confirm that the teletherapy unit is initialized with a predetermined set of parameters, including predetermined tolerances. While such systems improve the reproduceability of the machine parameters in a series of treatments, they provide no imaging capability or control over the proper patient positioning and/or teletherapy beam alignment. These systems are quite expensive, at times costing from $80,000 to $100,000 to equip one accelerator used to generate radiation.
X-ray film systems have been used in portal imaging, but the resolution of the images obtained is generally poor due to the large size of the radiation source. Even more significant in high energy radiation imaging, the x-ray film has poor dynamic range. Such film systems further generally lack the capability to use computer enhancement processes and techniques to improve the contrast of the image. In the high energy radiation therapy applications, where there is a lack of differential radiation absorption between soft tissue and bone, the x-ray film systems have substantial performance drawbacks. The film systems are also labor intensive, requiring many steps to be performed manually in obtaining film images.
It is desirable in imaging radiation portals during radiation therapy sessions to obtain two types of images. A first "localization" image is obtained in order to ensure proper positioning of the patient within the portal or target area. Short bursts of radiation are employed prior to the actual treatment session and the position of the patient may be adjusted upon review of the "localization" image data. It is also desirable to obtain a second image, termed a "verification" image, which is a single image of the target area obtained for the entire treatment session, in order to provide a record image of that session.
The apparatus and method disclosed in U.S. Pat. No. 4,365,341, to Lam, employs a solid-state detector which is used to detect intermittent high-energy pulses in an attempt to ensure proper patient positioning during the treatment session. Solid-state detectors, such as those disclosed in Lam, possess at least two disadvantages when employed in a high-energy detection system of this type. Use of a matrix or linear array of detectors will generally not be feasible for use in imaging from a cost standpoint, even when using elements 2 mm in diameter (assuming circular detectors are employed) which yield a 4 mm spatial resolution. A 50 cm.times.50 cm imaging array, for example, would require a matrix of 62,500 individual detectors. This number could be reduced by employing a linear array and translating the array as indicated in the Lam patent, however, a large number of detectors would still be required. Perhaps more importantly, a complete scan of the target area using such a device in order to obtain an image of the entire target would be very time consuming, rendering such a system impractical for clinical application. Additionally, solid state detectors are susceptible to radiation damage, and are therefore not practical for repeated use in detecting a high energy therapeutic beam. Damage from one week of exposure to such radiation, for example, could cause significant changes in the sensitivity of the solid-state detector elements.
Attempts have also been made at obtaining real time imaging during treatment using a fluorescent screen and video camera. Such systems produce images comparable to those obtained by x-ray film, and have the advantage over film that the image contrast may be enhanced by computer. The large size of such a fluorescent screen imaging device is a drawback in that its applications are limited to only certain angles when used in conjunction with a teletherapy machine.
Ion chambers of widely differing designs have heretofore been used for detecting radiation in various systems. Ion chambers previously disclosed as being used in connection with high-energy x-ray therapy equipment have generally been limited to detection of incident radiation in order to assist in aligning the beam of radiation. As seen in U.S. Pat. No. 3,955,089, to McIntyre et al, an x-ray therapy application, the ion chambers are used merely to detect the presence of radiation, and are used to surround the incident beam of radiation in order to assist in steering the beam into proper alignment. The ion chambers in this patent are not used to provide an imaging capability, as they are disposed on the incident beam side of the patient undergoing treatment. Overall, prior devices and methods have not provided those practicing in the radiation oncology field, and more particularly those directly involved in conducting radiation therapy, with good quality, low cost imaging of the radiation portal without also providing significant disadvantages associated with the systems.
It is therefore an important object of the present invention to provide an apparatus for obtaining images and visualizing the treatment portal during radiation therapy sessions.
It is a further important object of the present invention to provide an apparatus for obtaining images of the treatment portal which employs a detector comprising a plurality of ion chambers in a parallel array in combination with means for rotating the strip detector to collect image data over the area of the treatment portal.
It is a further important object of the present invention to provide an apparatus for obtaining images of the treatment portal which employs an accurate stepping motor as the means for rotating the detector, and further providing means for measuring small ionization currents generated in the array of ion chambers.
It is a further object of the present invention to provide ion chambers having parallel opposed electrodes and containing an ionizing medium of a liquid fluorocarbon material.
It is another important object of the present invention to provide a method for imaging the treatment portal and target during radiation therapy for obtaining localization and verification images which comprises the steps of generating a high energy radiation beam, directing the beam toward an area of the body to be treated, thereby creating a radiation portal, obtaining readings of absorbed radiation from the radiation target area using an array of ion chambers as a detector, measuring the ionization current at each ion chamber using a repetitive integral measurement technique, and reconstructing an image from the individual measurements by employing a convolution filtered back-projection technique or other reconstructive technique.